Heater apparatus of a hot gas external combustion piston engine

ABSTRACT

The working gas of the engine passes successively through the tubes of at least two parallel arrays of tubes and a heating gas passes successively through the interstices of the arrays, not necessarily in the same sequence. To equalize the thermal loading of the upstream and downstream arrays, referring to the stream of the heating gas, the open space between the tubes of the upstream array and/or the flow velocity inside the tubes of the upstream array is made higher than that inside the tubes of the downstream array. These flow velocity relations are provided by differences in tube diameter, differences in the number of tubes, configuration of heat transfer fins alone or in combination with each other or in combination with other features such as staggering of one array with respect to the other. The downstream array may be subdivided into two or more downstream arrays. An arrangement of two interleaved pairs of arrays is also shown.

Kuhlmann et al.

Feb. 4, 1975 HEATER APPARATUS OF A HOT GAS EXTERNAL COMBUSTION PISTON ENGINE Inventors: Peter Kuhlmann, Augsburg; Franz Beschorner, Neusass, both of Germany Assignee: Maschinent'abrik-Augsburg- Nurnberg Aktiengesellschaft, Augsburg, Germany Filed: Sept. 26, 1973 Appl. No.: 400,883

Foreign Application Priority Data Oct, 6, 1972 Germany 2249117 US. Cl. 60/522 Int. Cl. F011) 1/00 Field of Search 60/522; 165/176, 146;

References Cited UNITED STATES PATENTS 6/1880 Woodbury et al. 60/522 4/1917 Radigver 122/235 C 10/1960 Riley et al 165/146 X Primary Examiner-Martin P. Schwadron Assistant Examiner-H. Burks, Sr. Attorney, Agent, or FirmWilliam R. Woodward [57] ABSTRACT The working gas of the engine passes successively through the tubes of at least two parallel arrays of tubes and a heating gas passes successively through the interstices of the arrays, not necessarily in the same sequence. To equalize the thermal loading of the upstream and downstream arrays, referring to the stream of the heating gas, the open space between the tubes of the upstream array and/or the flow velocity inside the tubes of the upstream array is made higher than that inside the tubes of the downstream array, These flow velocity relations are provided by differences in tube diameter, differences in the number of tubes, configuration of heat transfer fins alone or in combination with each other or in combination with other features such as staggering of one array with re spect to the other, The downstream array may be subdivided into two or more downstream arrays. An arrangement of two interleaved pairs of arrays is also shown.

16 Claims, 10 Drawing Figures HEATER APPARATUS OF A HOT GAS EXTERNAL COMBUSTION PISTON ENGINE Cross-references to related applications: Grossmann et al. Ser. No. 315,930, Dec. 18, 1972 now U.S. Pat. No. 3,802,198: Double-acting Hot Gas Multi-cylinder Piston Engine Aupor et al. Ser. No. 317,778, Dec. 22, 1972 now U.S. Pat. No. 3,795,112:

Hot Gas Cylinder-Piston Apparatus Tusche Scr. No. 346,107, Mar. 29, 1973 now U.S.

Pat. No. 3,795,]02:

Double-Acting Reciprocating Hot Gas External Combustion Cylinder-Piston Engine This invention relates to hot gas piston engines of the external combustion type, and more particularly to multicylinder double acting engines in which the working gas moves between hot and cold chambers through a regenerator. More specifically this invention concerns the heating tube arrays of such an engine where the gas which expands in the hot chamber is subjected to heat in passing through an array of heating tubes.

Multicylinder engines of the type here concerned are described in U.S. application Ser. No. 315,930, filed Dec. 18, 1972 now U.S. Pat. No. 3,802,198.

In German Pat. No.. 806,740 an arrangement of a double array of heating tubes with the tubes of one array coupled to those and the heating gas passing through one array and then the other is shown, in which both arrays are circular. In the engine there disclosed there is a centrally disposed burner working from above and the hot gases produced by it are deflected from the axial direction and made to flow radially out through the inner heating tube array and then the outer array. In this case the inner array is the upstream array in terms of the direction of the flow of the heating gas. The number and dimensions of the heating tubes in each array is in this case the same. In consequence the heating tubes in the outer array are further apart than those of the inner, which means that the flow-through cross section of the outer array for the heating gas is greater than that of the inner array. This leads to a drop in the flow velocity of the heating gas from the inner array to the outer array and hence to a deterioration of the heat transfer coefficient that depends upon the flow velocity. Since the temperature of the heating gas likewise drops from the inside towards the outside, while the temperature of the working gas flowing inside the heating tubes does not change substantially, the temperature difference between the outer and inner surfaces of the heating tube, the next most important magnitude affecting the heat transfer, likewise deteriorates in the same direction.

In the apparatus just referred to, because of the diminishing efficiency of heat transfer from the inner to the outer array of heating tubes, a very much greater quantity of heat is applied to the upstream tube array(- than to the downstream array). Since inside the heating tubes of the inner and outer arrays substantially the same heat transfer conditions apply, the heating tubes of the upstream array are raised to a much higher temperature than those of the downstream array. In consequence there is a danger that the heating tubes of the upstream array will be overheated. On the other hand, the heat transfer capacity of the downstream array is very small so that the overall heat transfer efficiency of the heater is low.

It is an object of the present invention to provide a heater for the working gas of an external combustion engine of the multiple array type with approximately equal thermal loading of the heater tubes in the different arrays and thereby to increase the heat transfer capacity, and to do so at reasonable cost.

SUBJECT MATTER OF THE PRESENT INVENTION Briefly, the heater tube arrays are so constructed that the flow-through cross-sectional area for the heating gas provided by the upstream array is greater than that provided by the downstream array, or else the flowthrough cross section of the upstream array for the working gas is smaller for the tubes of the upstream array than for those of the downstream array, or else both of these features are used simultaneously. In consequence, in going from the upstream array to the downstream array there is an increase in the flow velocity of the heating gas and/or a decrease in the flow velocity of the working gas. As a result of the heat transfer coefficient at the outside of the heating tubes of the downstream array is improved and, on the other hand, the heat transfer coefficient at the inside of the heating tubes of the upstream array is likewise improved.

A temperature drop in the heating gas in the down stream direction is of course inevitable. This drop of the driving temperature difference of the system is to a great extent compensated by providing very good inner cooling of the heating tubes of the upstream array and by providing very efficient heating of the outside of the heating tubes of the downstream array. In this manner nearly the same wall temperature of all heating tubes can be obtained, so that during operation there is substantially the same thermal loading of each heating tube array. According to this invention, an increase of the thermal loading of the heater as a whole increases the loading of all heating tubes equally, so that the thermal loading can be more safely increased to the limit of the thermal capacity of the individual tubes and an optimum utilization of the thermal capacity of all components of the heater can be obtained. Such an operation, because it contains no components not fully used and does not require some parts to be made for much higher capacities than the others, can also save weight, space and cost.

In most forms of the invention the heating tube arrays are in straight lines, which is particularly suitable for engines with cylinders and regenerators arranged in straight lines. In one form of the invention the spacing between the outside of the tubes in the upstream array is greater than in the downstream array. This may be accomplished in various ways, the diameter of the tubes of the downstream array may be greater, or they may be the same and there may be more of them, with the couplings between the upstream and downstream arrays using a branching section, or the tubes may have an oval cross section presenting their narrow diameters to the flow of gas in the upstream array and their broad diameters in the downstream array. In some forms of the invention there are more than two arrays. In one form there is one array of tubes connected to the hot chambers of the cylinders and two arrays, one behind the other, of tubes connected to the regenerators. with the connections between the upstream and downstream arrays being branched so that each hot chamber is connected to a regenerator in each row. In another form of the invention there are two arrays of tubes connected to hot chambers of cylinders each connected with an array of tubes connected to regenerators, the upstream array of tubes connected to hot chambers of cylinders being connected to the more upstream array of the tubes connected to regenerators.

The invention is further described in connection with specific embodiments by way of illustration with reference to the annexed drawings in which:

FIG. 1 is a diagrammatic view from above, partly broken away, of a first embodiment of the invention;

FIG. 2 is a vertical cross section along the line Il-II of FIG. 1;

FIG. 3 is a diagrammatic plan view of a second embodiment of the invention;

FIG. 4 is a vertical cross section along the line lV-IV of FIG. 3;

FIG. 5 is a diagrammatic plan view of a triple array heater;

FIG. 6 is a vertical cross section along the line Vl-VI of FIG. 5;

FIG. 7 is a vertical cross section along the line VII VII of FIG. 5',

FIG. 8 is a diagrammatic plan of a heater with four rows of tubes;

FIG. 9 is a vertical cross section along the line IXIX of FIG. 8; and

FIG. 10 is a vertical cross section along the line X-X of FIG. 8.

FIG. I is a diagram ofa heater with two rows oftubes in which the tubes of the downstream row are of larger diameter and hence also of closer external spacing. The direction of flow of the heating gas is shown by the arrows I. The two rows of tubes are arranged in straight rows 2 and 3 lined up perpendicular to the direction of heating gas flow. FIG. 2 shows how a heating tube 4 of the upstream row 2 is connected by a U-shaped tube 9 to a heating tube 7 of the downstream row 3. As shown at the top of FIG. I each of the tubes ofone row is connected to one of the tubes of the other.

As shown in FIG. 2 the heating tubes of the upstream array 2, such as the tube 4, are connected to a hot working chamber 6 of a cylinder 5 of the engine, whereas the heating tubes of the downstream array 3, such as the tube 7 are connected at their lower ends to a regenerator unit 8. The burner 10, shown diagrammatically in FIG. 2, is disposed on the upstream side of the tube array 2 and projects hot gases on to the two arrays of tubes in succession, first impinging on the tubes connected to the hot working chambers of the engine and then on the tubes connected with the regenerators.

The hot gas flowing through the gaps in the two arrays of tubes transfers heat to the working gas flowing back and forth between the hot working chambers of the engine and the regenerators. In order to provide, in accordance with the invention, the desired relations of flow velocity of the heating gas through the upstream and downstream arrays, the tubes of the upstream array 2 have a smaller outer diameter d and a larger separation distance 0. whereas the tubes of the downstream array have a larger outer diameter D and a smaller separation distance b. as shown in FIG. 1. The ratio of outer diameter to inner diameter is the same for the tubes of both arrays. The two arrays are of the same length and have the same number of tubes, but one is staggered with respect to the other, so that a heating tube of the downstream array is lined up, in the direction of flow of heating gas. with a gap of the upstream array.

As the result of the small gaps in the downstream array 3 there is greater flow velocity of the heating gas around the tubes of that array, as well as greater area of contact. resulting in a higher heat transfer coefficient for the outer surface of these tubes. Since the inner diameters of the tubes of the two arrays are proportional to the outer diameters, the cross-sectional area f of the inside of the tube 4 is smaller than the cross-sectional area F of the inside of the tube 7, but since in a given unit of time, with the same quantity of the working gas passing through both tubes, the working gas is forced through the tubes of the upstream array 2 at a higher velocity than that with which it flows through the tubes of the downstream array 3. In consequence there is a very good heat transfer at the inner surface of the upstream tubes and hence a very good cooling of the tube 4 by the working gas. The staggered arrangement of the two arrays provides good impingement of the heating gas on the tubes of the downstream array.

In this example the tubes are arrayed in straight lines, By the arrangement of the upstream array 2 in a circular arc bulging eonvexly towards the burner I0 and the provision of the downstream array 3 on a concentric arc of smaller radius. the separation distance a between the tubes of the upstream array could be made still greater in relation to the separation distance 1 between the tubes of the downstream array.

In order to avoid undue dissipation of energy in the flow of the working gas in the transition from the smaller diameter d to the larger D it is important to pro vide a tapered transition section in the piping. In the ex ample shown in FIG. 2, this taper is provided by a tapered narrowing of the upper part of the tube 7, as shown at 11 on FIG. 2, just below its connection to the double elbow pipe 9.

FIGS. 3 and 4 show an embodiment of the invention in which the improvement of the heat transfer coeffi cients is provided by the use of an upstream array containing fewer tubes than the downstream array. Each heating tube 12 of the upstream tube array 13 is connected by a branching pipe 16 to two heating tubes 14 of equal size disposed in the downstream array IS. The particular advantage of this arrangement lies in the fact that all of the heating tubes of both arrays can be ex' actly alike. In that manner production costs can be re duced and parts storage simplified.

The effectiveness of the arrangement shown in FIG. 3 can, for example, be still further increased if heater tubes of oval cross section are used, in which case the tubes of the upstream array would be arranged with their smaller diameters facing the hot gas flow, while the tubes of the downstream array would have reduced spacing, not only because of their greater number, but also because of being oriented with their larger diameters transverse to the direction of flow of the heating gas. The various features described with reference to FIGS. l to 4 can be combined with each other and also with other measures according to the invention.

In FIGS. 5 and 6 a further example of a heater arrangement according to the invention is shown. The heater tubes 17 of the upstream tube array 18 are in this case connected. each through a distributing pipe 19 having three connections. to the heating tubes 20 and 21 of two downstream tube arrays 22 and 23 respectively. The upstream array 18 can, for example, be connected to the hot working chambers of the engine and the two downstream arrays to the regenerators. The reverse arrangement, is however also possible. The direction of flow of the working gas in the tubes of the array 18 is in any event always opposite to the flow of the working gas in the downstream arrays 22 and 23. The use of heater tubes that are all alike will provide in the case of such a three array heater an increase of the velocity of flow of the working gas in the tubes of the upstream array on account of the coupling of each tube of the upstream array with two downstream tubes, one from each array. In the heater design shown in FIGS. 5 and 6 the first downstream array 22 has the same number of heater tubes as the upstream array 18, but the second downstream array 23 has twice as many heater tubes. As shown in FIG. 7, the heater tubes 2| of the array 23 are made in pairs, with each pair being made in one piece and having a single connection to the distributing pipe 19. In that portion near the distributing pipe 19 the heater tube pair unit has a forked portion connecting both tubes of the pair to the distributing pipe 19 through an extended mouthpiece 24. These last heater tube units are so arranged that their greatest projected surface faces the heating gas stream, reducing the open cross-sectional area of the array to a minimum. In order to increase the open cross-sectional area for the heating gas, for example in the upstream array 18, similar double heater tube units with their narrow side facing the heating gas flow could be used. In order to increase the heat transfer surfaces as well as to narrow the open crosssectional area provided by the array for the heating gas, the heating tubes 20 and 21 of the two downstream arrays 22 and 23 are provided with ribs 25.

In FIG. 5 the heater tubes of the upstream array are shown as of larger diameter than those of the downstream arrays. This diameter difference is in the opposite sense from that which would follow the principles of this invention, but it is outweighed by far in this case by the much greater number of tubes in the combined downstream arrays, compared to the upstream array. and their extensive fin structures 25.

FIG. 8 shows a four-row tube heater in which two rows of heater tubes are connected with the hot work chambers of the engine and the other two rows of tubes are connected with the regenerators. In this way the cylinder spacing in a multicylinder motor can be kept small and likewise the overall bulk of the motor as a whole. The array farthest upstream, the tube row 26, is connected with the third row of tubes 27, whereas the second row of tubes 28 is connected with the fourth and farthest downstream row 29. The number of tubes in each of the two farthest downstream rows 27 and 29, for example connected to the regenerators of the engine, is twice as great as the number of tubes in the upstream rows 26 and 28 with which they are connected and which are connected, for example, to the hot working chambers of the engine cylinders. in the illustrated case the diameter of all of the tubes used is the same. It is clear from the previous discussion, however, that tubes of different diameters and/or of oval cross section could also be used in this arrangement.

FIG. 9 shows U-shaped pipe connections 30 connecting the tubes of two different arrays. On the downstream side these connecting pipes 30 connect to cross pipes or manifolds 31 for the downstream arrays 27 and 29 respectively, extending over the full length of the array. These manifold pipes 3| to which all of the U- shaped connecting pipes 30 are connected make it possible to provide the optimum number of heater tubes for the downstream arrays for the particular case. In addition, variations in the flow through the pipes can be equalized for the entire heater by this arrangement.

FIG. 10 shows a longitudinal cross section of a portion of the manifold pipe 31, showing the connection with the U-shaped pipes 30 preferably opposite an in terval between connections to the heater tubes, for a more even distribution of flow and reduction of energy dissipation in the flow. In the arrangement shown there is one U-tube for every two heater tubes connected to the manifold pipe 3]. Ribs 32, as shown on one pair of heater tube arrays in FIG 9 can be provided for increasing the heat transfer surfaces. As also shown in FIG. 9, the ribs 32 extend out further from the heater tube in the downstream array 29 than in the upstream array 28, for the reasons already previously discussed. The plan view of the ribs 32 is shown in FIG. 8. it may be observed that it is also possible to associate more than one downstream array of tubes with each of the upstream arrays 26 and 28, thus combining the features of FIG. 5 and FIG. 8.

It is similarly to be understood that the arrangement of FIG. 5 could be modified by using staggering of the arrays and differences in the diameter of the tubes to obtain the benefits of the invention as well as or instead of use of a larger number of tubes as shown in the array 23.

One of the advantages of the interleaved four array system shown in FIG. 8 is that the pressure drop in all the connecting elbows is substantially the same. In this system, because of the large number of heater tubes, the individual heater tubes can be very short and the heater as a whole can likewise be very short in its dimension parallel to the axes of the tubes, so that the pressure losses in the working gas can be held particularly low, to the benefit of the efficiency of the motor. and, in addition, it is also easier to heat the entire length and height of the heater tube arrays evenly by a burner producing the hot gas stream impinging on the heater tubes. Local overheating can thus be more easily avoided.

It is of no importance for the effectiveness of the present invention whether the burner acting on the heater arrays is disposed on the regenerator side or on the side of the hot work chambers of the engine cylinders. In the case of in-line motors in which the regenerators associated with the respective cylinders are arranged alternately on each side of the cylinder row, there is complete freedom of design regarding location of the burner that fires the heater. Since in particular the four-row heater has relatively small dimensions, in some cases a single burner can be used for heating two or more heaters arranged next to each other.

Although the invention has been described with respect to particular illustrative embodiments. it is to be understood that the features of the various embodiments shown can be differently combined as above mentioned in a few instances. and other variations and modifications may be made within the inventive concept.

We claim:

l. A hot gas piston engine of the external combustion type in which the working gas is heated, in passing to and fro between a plurality of regenerators and a plurality of work chambers through heating tubes, by a heating gas passing around the outside of said tubes, comprising:

two arrays of substantially straight and parallel heating tubes, one being located behind the other in the direction of flow of said heating gas and both arrays being disposed transverse to said direction of flow, including a first array leading said working gas directly to and from said hot work chambers without substantial change of direction of flow between said hot work chambers and the tubes of said first array and a second array leading said working gas directly to and from said regenerators without substantial change of direction of flow between said regenerators and the tubes of said second array; connecting duct means for guiding said working gas back and forth between said first and second arrays with not substantially more than l80 of change of direction of flow of said working gas;

that one of said arrays of heating tubes which is upstream in the path of flow of heating gas (2,l3,l8,26,28) having a greater open crosssectional area for the How of heating gas than the other of said arrays (3,]5.22,23,27,29) which is downstream in the path of flow of heating gas.

2. A hot gas piston engine as defined in claim 1 in which at least the said upstream array consists of a single row of heating tubes.

3. A hot gas piston engine as defined in claim 1 in which the said upstream array has a smaller open crosssectional area for the flow of said working gas than the downstream array.

4. An engine as defined in claim 1 in which the clearance spacing between individual heating tubes of said upstream array (2, l3, 18, 26. 28) is greater than that between said heating tubes of said downstream array (3,15, 23, 27, 29).

5. An engine as defined in claim 3. in which the clearance spacing between individual heating tubes of said upstream array (2, l3, 18, 26, 28) is greater than that between said heating tubes of said downstream array (3.15, 23, 27, 29), and in which said heating tubes (4) of said upstream array (2) have smaller outer and inner diameters than said heating tubes (7) of said downstream array (3).

6. An engine as defined in claim 5 in which said con necting means includes tapered transition tube section (ll) for smooth transition from the smaller diameter of the said tubes of said upstream array to the larger diameter of the said tubes of said downstream array.

7. An engine as defined in claim 3 in which said upstream array of tubes 13, 26, 28) has a smaller number of heating tubes than said downstream array (15. 27, 29).

8. An engine as defined in claim Sin which said connecting duct means includes manifold means (31) for connecting the smaller number of tubes of said upstream array (26, 28) with the greater number of tubes of said downstream array (27, 29).

9. An engine as defined in claim 3 in which. the downstream array includes a plurality of rows of tubes and each heating tube of said upstream array (18) in connected to at least one tube of each row of said downstream array (22, 23) and in which the direction of flow of the working gas in the downstream array is in a direction opposite to its direction of flow in the upstream array.

10. An engine as defined in claim 9 in which said connecting means include branching sections and each heating tube (l7) of said upstream array (18) is connected by a branching section (19) with downstream heating tubes (20. 2]) of said downstream tube array (22, 23) which are not all in the same row ofthe downstream array (22, 23).

ll. An engine as defined in claim 7 in which said connecting means comprise branching tubes, both of said arrays consist ofa single row of tubes, and each heating tube of the upstream array (13) is connected over a branching tube (16) with a plurality of heater tubes of the downstream array (l5).

12. An engine as defined in claim 1 in which both of said arrays comprise a plurality of rows of tubes (26, 27, 28, 29) one behind the other in the direction of flow of said heating gas, and in which the tubes of the more upstream rows of the upstream array are connected by said connecting means to tubes of more upstream rows of the downstream array and in which the tubes of the more downstream rows of the upstream array are connected by said connecting means to tubes of more downstream rows of the downstream array.

l3. An engine as defined in claim 12 in which at least the upstream array consists of two rows of tubes (27, 28) and the upstream row of tubes (26) thereof is connected by said connecting means to tubes (27) of said downstream array which are upstream of the tubes (29) of the downstream array that are connected to tubes of the downstream row (28) of the upstream ur' ray, and in which, further, the open cross-sectional area for passage of the heating gas presented by the upstream array and that presented by the tubes of the downstream array (27) connected thereto are respec tively greater than the cross-sectional area for such passage presented by the downstream row (28) of the upstream array and by the tubes of the downstream array (29) connected thereto.

14. An engine as defined in claim 1 in which the tubes of each of said arrays are arranged in at least one straight row.

15. An engine as defined in claim I in which said heating tubes of said arrays are provided with heat transfer ribs (32) which are of greater size on the downstream tubes.

16. An engine as defined in claim 1 in which said heating tubes are of oval cross section and are aligned in the upstream array with the major cross-sectional diameter parallel to the flow of heating gas and in the downstream array with the major cross-sectional axis transverse to the flow of heating gas. 

1. A hot gas piston engine of the external combustion type in which the working gas is heated, in passing to and fro between a plurality of regenerators and a plurality of work chambers through heating tubes, by a heating gas passing around the outside of said tubes, comprising: two arrays of substantially straight and parallel heating tubes, one being located behind the other in the direction of flow of said heating gas and both arrays being disposed transverse to said direction of flow, including a first array leading said working gas directly to and from said hot work chambers without substantial change of direction of flow between said hot work chambers and the tubes of said first array and a second array leading said working gas directly to and from said regenerators without substantial change of direction of flow between said regenerators and the tubes of said second array; connecting duct means for guiding said working gas back and forth between said first and second arrays with not substantially more than 180* of change of direction of flow of said working gas; that one of said arrays of heating tubes which is upstream in the path of flow of heating gas (2,13,18,26,28) having a greater open cross-sectional area for the flow of heating gas than the other of said arrays (3,15,22,23,27,29) which is downstream in the path of flow of heating gas.
 2. A hot gas piston engine as defined in claim 1 in which at least the said upstream array consists of a single row of heating tubes.
 3. A hot gas piston engine as defined in claim 1 in which the said upstream array has a smaller open cross-sectional area for the flow of said working gas than the downstream array.
 4. An engine as defined in claim 1 in which the clearance spacing between iNdividual heating tubes of said upstream array (2, 13, 18, 26, 28) is greater than that between said heating tubes of said downstream array (3, 15, 23, 27, 29).
 5. An engine as defined in claim 3, in which the clearance spacing between individual heating tubes of said upstream array (2, 13, 18, 26, 28) is greater than that between said heating tubes of said downstream array (3, 15, 23, 27, 29), and in which said heating tubes (4) of said upstream array (2) have smaller outer and inner diameters than said heating tubes (7) of said downstream array (3).
 6. An engine as defined in claim 5 in which said connecting means includes tapered transition tube section (11) for smooth transition from the smaller diameter of the said tubes of said upstream array to the larger diameter of the said tubes of said downstream array.
 7. An engine as defined in claim 3 in which said upstream array of tubes (13, 26, 28) has a smaller number of heating tubes than said downstream array (15, 27, 29).
 8. An engine as defined in claim 5 in which said connecting duct means includes manifold means (31) for connecting the smaller number of tubes of said upstream array (26, 28) with the greater number of tubes of said downstream array (27, 29).
 9. An engine as defined in claim 3 in which, the downstream array includes a plurality of rows of tubes and each heating tube of said upstream array (18) in connected to at least one tube of each row of said downstream array (22, 23) and in which the direction of flow of the working gas in the downstream array is in a direction opposite to its direction of flow in the upstream array.
 10. An engine as defined in claim 9 in which said connecting means include branching sections and each heating tube (17) of said upstream array (18) is connected by a branching section (19) with downstream heating tubes (20, 21) of said downstream tube array (22, 23) which are not all in the same row of the downstream array (22, 23).
 11. An engine as defined in claim 7 in which said connecting means comprise branching tubes, both of said arrays consist of a single row of tubes, and each heating tube of the upstream array (13) is connected over a branching tube (16) with a plurality of heater tubes of the downstream array (15).
 12. An engine as defined in claim 1 in which both of said arrays comprise a plurality of rows of tubes (26, 27, 28, 29) one behind the other in the direction of flow of said heating gas, and in which the tubes of the more upstream rows of the upstream array are connected by said connecting means to tubes of more upstream rows of the downstream array and in which the tubes of the more downstream rows of the upstream array are connected by said connecting means to tubes of more downstream rows of the downstream array.
 13. An engine as defined in claim 12 in which at least the upstream array consists of two rows of tubes (27, 28) and the upstream row of tubes (26) thereof is connected by said connecting means to tubes (27) of said downstream array which are upstream of the tubes (29) of the downstream array that are connected to tubes of the downstream row (28) of the upstream array, and in which, further, the open cross-sectional area for passage of the heating gas presented by the upstream array and that presented by the tubes of the downstream array (27) connected thereto are respectively greater than the cross-sectional area for such passage presented by the downstream row (28) of the upstream array and by the tubes of the downstream array (29) connected thereto.
 14. An engine as defined in claim 1 in which the tubes of each of said arrays are arranged in at least one straight row.
 15. An engine as defined in claim 1 in which said heating tubes of said arrays are provided with heat transfer ribs (32) which are of greater size on the downstream tubes.
 16. An engine as defined in claim 1 in which said heating tubes are of oval cross section and are aligned in the upstream array with the major cross-sectional diametEr parallel to the flow of heating gas and in the downstream array with the major cross-sectional axis transverse to the flow of heating gas. 